This problem can also be called
This problem can also be called finding the domination number of a grid graph.
In Computing the Domination Number of Grid Graphs by Samu Alanko, he highlights many of the formulas known for different-sized grid graphs. In particular, he quotes Hare's work "Algorithms for Grids and Grid-Like Graphs" for a m x n grid graph for m = 7, 8. Using the formula listed for m = 8 with $m\leq n$, the domination number $\gamma$ is $$\gamma_{8,n}=\Bigl\lfloor\frac{15n+14}{8}\Bigr\rfloor$$ Plugging in 8 for n, we get
Plugging in 8 for n, we get
16 which is the smallest number needed to cover an 8x8 grid with pentominoes :)